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Howlett , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, kernel-team@meta.com, Usama Arif Subject: [PATCH v4 10/11] mm: install PMD swap entries on swap-out Date: Mon, 13 Jul 2026 06:35:45 -0700 Message-ID: <20260713133613.2707815-11-usama.arif@linux.dev> In-Reply-To: <20260713133613.2707815-1-usama.arif@linux.dev> References: <20260713133613.2707815-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 269F980010 X-Stat-Signature: upiawbo7ssqr94o4kxr9zfjmf5btzi66 X-HE-Tag: 1783949850-665834 X-HE-Meta: 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 +H688nbC 8i+hr/m8v6cijyjqhbs0yhOGhEP2NlctCFYLH9emvdSjcZXSfs17RbosOCXdtbzGhcOk1Ryjr4Eacsyw7eJkuZXQ5VPieTDjcrh9oFsGQRSKKkW+HNNLebH8mSXFCGUYtRgXCET0XfaGj7SIWOLZJrs7yQqfqpvn02R7yhwH4SYPd5zv0o7K6JWlgpNkqVvoyJkq2HQUIr6ocNfk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Reclaim today splits a PMD-mapped anonymous THP into 512 PTE swap entries before unmap, losing the huge mapping across the swap round-trip and forcing khugepaged to rebuild it later. The contiguous swap range was already secured when the folio was added to the swap cache (a non-contiguous allocation would have split the folio earlier), so the PMD can be replaced by a single PMD-level swap entry instead. This patch mirrors the existing PTE swap-out path at PMD granularity: - shrink_folio_list() drops TTU_SPLIT_HUGE_PMD for PMD-mappable swapcache folios. zswap is handled by the PMD swap-in users: if any covered slot currently has a zswap entry, they split the PMD swap entry and fall back to the per-PTE path. - try_to_unmap_one() now has a PMD branch that calls set_pmd_swap_entry() and adjusts MM_ANONPAGES / MM_SWAPENTS by HPAGE_PMD_NR before walk_done. TTU_SPLIT_HUGE_PMD remains the fallback. - set_pmd_swap_entry() is the installer. Mirroring the PTE swap-out sequence at PMD granularity, it clears the present mapping (keeping the original for rollback), bumps the swap_map refcount for the folio's 512 slots, transfers the exclusive state in the swap entry, propagates the dirty bit to the folio so writeback is not lost, and installs a swap PMD that preserves the original soft-dirty / uffd-wp / exclusive bits. Any failing step rolls back the present mapping. The swap entry value matches what 512 PTE swap entries would encode, so swap_map refcounting is unchanged: each of the 512 slots carries a count of 1, released individually on later split or together on swap-in. Signed-off-by: Usama Arif --- include/linux/huge_mm.h | 2 + include/linux/vm_event_item.h | 1 + mm/huge_memory.c | 80 +++++++++++++++++++++++++++++++++++ mm/rmap.c | 19 +++++++++ mm/vmscan.c | 9 +++- mm/vmstat.c | 1 + 6 files changed, 111 insertions(+), 1 deletion(-) diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index b6ab28ff7eee..6d294d446f7b 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -536,6 +536,8 @@ vm_fault_t do_huge_pmd_device_private(struct vm_fault *vmf); #ifdef CONFIG_THP_SWAP vm_fault_t do_huge_pmd_swap_page(struct vm_fault *vmf); +int set_pmd_swap_entry(struct page_vma_mapped_walk *pvmw, + struct folio *folio); #else static inline vm_fault_t do_huge_pmd_swap_page(struct vm_fault *vmf) { diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h index 03fe95f5a020..7267c06674c0 100644 --- a/include/linux/vm_event_item.h +++ b/include/linux/vm_event_item.h @@ -108,6 +108,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT, THP_ZERO_PAGE_ALLOC_FAILED, THP_SWPOUT, THP_SWPOUT_FALLBACK, + THP_SWPOUT_PMD, #endif #ifdef CONFIG_BALLOON BALLOON_INFLATE, diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 60f42d136945..2f5c01a043fe 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -5544,3 +5544,83 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new) trace_remove_migration_pmd(address, pmd_val(pmde)); } #endif + +#ifdef CONFIG_THP_SWAP +/** + * set_pmd_swap_entry() - Replace a PMD mapping with a PMD-level swap entry. + * @pvmw: Page vma mapped walk context, must have pvmw->pmd set and + * pvmw->pte NULL (i.e. PMD-mapped). + * @folio: The folio being swapped out. Must be in the swap cache. + * + * This installs a PMD-level swap entry in place of a present PMD mapping, + * avoiding the need to split the PMD into PTE-level swap entries. + * + * Return: 0 on success, negative error code on failure. + */ +int set_pmd_swap_entry(struct page_vma_mapped_walk *pvmw, + struct folio *folio) +{ + struct vm_area_struct *vma = pvmw->vma; + struct mm_struct *mm = vma->vm_mm; + unsigned long address = pvmw->address; + unsigned long haddr = address & HPAGE_PMD_MASK; + struct page *page = folio_page(folio, 0); + bool anon_exclusive; + pmd_t pmdval; + swp_entry_t entry; + pmd_t pmdswp; + + if (!(pvmw->pmd && !pvmw->pte)) + return 0; + + VM_BUG_ON_FOLIO(!folio_test_swapcache(folio), folio); + VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio); + + if (unlikely(folio_test_swapbacked(folio) != + folio_test_swapcache(folio))) { + WARN_ON_ONCE(1); + return -EBUSY; + } + + flush_cache_range(vma, haddr, haddr + HPAGE_PMD_SIZE); + + pmdval = pmdp_invalidate(vma, haddr, pvmw->pmd); + + /* Update high watermark before we lower rss */ + update_hiwater_rss(mm); + + if (folio_dup_swap(folio, NULL) < 0) { + set_pmd_at(mm, haddr, pvmw->pmd, pmdval); + return -ENOMEM; + } + + /* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */ + anon_exclusive = PageAnonExclusive(page); + if (anon_exclusive && folio_try_share_anon_rmap_pmd(folio, page)) { + folio_put_swap(folio, NULL); + set_pmd_at(mm, haddr, pvmw->pmd, pmdval); + return -EBUSY; + } + + mm_prepare_for_swap_entries(mm); + + if (pmd_dirty(pmdval)) + folio_mark_dirty(folio); + + entry = folio->swap; + pmdswp = softleaf_to_pmd(entry); + if (pmd_soft_dirty(pmdval)) + pmdswp = pmd_swp_mksoft_dirty(pmdswp); + if (pmd_uffd(pmdval)) + pmdswp = pmd_swp_mkuffd(pmdswp); + if (anon_exclusive) + pmdswp = pmd_swp_mkexclusive(pmdswp); + set_pmd_at(mm, haddr, pvmw->pmd, pmdswp); + + folio_remove_rmap_pmd(folio, page, vma); + folio_put(folio); + + count_vm_event(THP_SWPOUT_PMD); + return 0; +} +#endif /* CONFIG_THP_SWAP */ diff --git a/mm/rmap.c b/mm/rmap.c index e854679553b9..b431d52cc2e3 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2079,6 +2079,25 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, goto walk_abort; } +#ifdef CONFIG_THP_SWAP + /* + * If the folio is in the swap cache and we're not + * asked to split, install a PMD-level swap entry. + */ + if (!(flags & TTU_SPLIT_HUGE_PMD) && + folio_test_anon(folio) && + folio_test_swapcache(folio)) { + if (set_pmd_swap_entry(&pvmw, folio)) + goto walk_abort; + + add_mm_counter(mm, MM_ANONPAGES, + -HPAGE_PMD_NR); + add_mm_counter(mm, MM_SWAPENTS, + HPAGE_PMD_NR); + goto walk_done; + } +#endif + if (flags & TTU_SPLIT_HUGE_PMD) { /* * We temporarily have to drop the PTL and diff --git a/mm/vmscan.c b/mm/vmscan.c index 986dde8e7429..ffc72a2f635b 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1322,7 +1322,14 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, enum ttu_flags flags = TTU_BATCH_FLUSH; bool was_swapbacked = folio_test_swapbacked(folio); - if (folio_test_pmd_mappable(folio)) + /* + * With THP_SWAP, PMD-mappable folios already in the + * swap cache can be unmapped with a PMD-level swap + * entry, avoiding the cost of splitting the PMD. + */ + if (folio_test_pmd_mappable(folio) && + !(IS_ENABLED(CONFIG_THP_SWAP) && + folio_test_swapcache(folio))) flags |= TTU_SPLIT_HUGE_PMD; /* * Without TTU_SYNC, try_to_unmap will only begin to diff --git a/mm/vmstat.c b/mm/vmstat.c index 3b5cb1031f72..08af18f2b4dc 100644 --- a/mm/vmstat.c +++ b/mm/vmstat.c @@ -1422,6 +1422,7 @@ const char * const vmstat_text[] = { [I(THP_ZERO_PAGE_ALLOC_FAILED)] = "thp_zero_page_alloc_failed", [I(THP_SWPOUT)] = "thp_swpout", [I(THP_SWPOUT_FALLBACK)] = "thp_swpout_fallback", + [I(THP_SWPOUT_PMD)] = "thp_swpout_pmd", #endif #ifdef CONFIG_BALLOON [I(BALLOON_INFLATE)] = "balloon_inflate", -- 2.53.0-Meta